1. Field of the Invention
The present invention relates to an expansion-molded article of a polyolefin resin, which has open voids, and is good in water permeability, air permeability and sound absorbing quality, and suitable for use, in particular, as a draining material, wall covering material, core material and the like, and a production process thereof.
2. Description of the Background Art
In recent years, water-permeable expansion-molded articles having open voids have come to be used as drainage-accelerating materials at places such as golf links, riding grounds and bridle paths in riding clubs, etc., of which high drainage performance is required, underlay materials for drainage of artificial lawn in roof gardens, and the like. Expansion-molded articles of this kind have been proposed in Japanese Patent Application Laid-Open Nos. 153026/1992 and 177723/1993.
However, the former molded article is formed by bonding foamed particles of a polystyrene to one another with a polyolefin resin. This molded article is produced by first mixing the foamed particles with an aqueous dispersion of the polyolefin resin, filling the resulting mixture into a mold to heat it and then cooling and solidifying it. Therefore, this process is complicated in production process, takes a long time to mold and is also poor in productivity compared with a process in which foamed particles are filling into a mold as they are, thereby molding them. In addition, this process also has involved a shortcoming that although the bond strength among the foamed particles depends on the strength of a film of the polyolefin resin formed on the surfaces of the foamed particles, this film is thin and tends to be broken, and so sufficiently high bond strength cannot be achieved among the foamed particles.
Besides, the molded article described in the latter is formed by filling a great number of foamed chips in an aspherical form, the length of the longest part of which is at least 2 cm, into a mold and then heating them, thereby fusion-bonding the chips to one another. Since the length of the longest part of the foamed chip is as long as at least 2 cm, scattering tends to occur in the size of voids defined among the foamed chips upon filling the chips into the mold, and so the filling density of the chips tends to vary with a difference in position among the chips filled in the mold. In addition, since the filling density varies every filling, it is difficult to uniformly fill the chips into the mold, and so difficulties are encountered on controlling the filling factor of the chips to a specific value. Therefore, it is difficult to control void content to a fixed value and to obtain a molded article having a uniform void content. Accordingly, it has been impossible to provide a molded article having uniform water permeability. Further, there has also been offered a problem that since the proportion of contact area among the chips is low, the bond strength among the chips is low, and so the whole expansion-molded article becomes brittle and easy to be broken.
On the contrary, an expansion-molded article of a polyolefin resin, which was previously proposed by the present applicants (Japanese Patent Application Laid-Open No. 224727/1991), is obtained by mutually fusion-bonding columnar foamed particles of the polyolefin resin, whose L/D is 2-10 wherein L is a length of the longest part of the foamed particle, and D is a sectional length of the largest barrel part, with their surfaces turned in irregular directions. This molded article has no need to separately use an adhesive for mutually bonding the foamed particles unlike the molded article described in Japanese Patent Application Laid-Open No. 153026/1992. Therefore, this process is simple in production process and excellent in productivity. Moreover, since the foamed particles are directly fusion-bonded to one another, breakage near interfaces among the foamed particles does not easily occur, and so the strength of the molded article is high. Besides, since the foamed particles are in the form of a column in which L/D is 2-10, and are simple in shape, they can be filled into the mold with a comparatively uniform density. Therefore, the void content of the resulting molded article is comparatively uniform at all positions of the molded article, and the void contents of individual molded articles are also comparatively constant.
However, if a molded article having sufficient water permeability is intended to provide in the molded article described in Japanese Patent Application Laid-Open No. 224727/1991, it is necessary to preset the pressure of steam upon molding the foamed particles low to prevent secondary expansion. As a result, the fusion-bonding strength among the foamed particles becomes low, and the compressive strength of the molded article is also lowered. If the molded article is intended to have high fusion-bonding strength among the foamed particles, and the pressure of steam upon molding is hence preset high on the contrary, even spaces to become voids are filled, resulting in a molded article insufficient in water permeability. It has hence been difficult to provide an excellent molded article which can satisfy both good water permeability in the molded article, and high fusion-bonding strength among the foamed particles and compressive strength of the molded article at the same time.
On the other hand, when a styrene resin is used as a base resin, an expansion-molded article with voids can be obtained with comparative ease by filling foamed particles thereof into a mold and heating them as described in Japanese Patent Application Laid-Open No. 134877/1994.
However, since there are the following points of difference between foamed particles of a polystyrene resin and foamed particles of a polyolefin resin, the technique for the polystyrene resin cannot be simply applied to the polyolefin resin. More specifically, the first point of difference between the foamed particles of the polystyrene resin and the foamed particles of the polyolefin resin is that temperatures at which fusion bonding is feasible are different, and so the latter foamed particles are required to heat at a higher temperature. The second point of difference is that the foamed particles of the polyolefin resin are poorer in secondary expandability than the foamed particles of the polystyrene resin. Incidentally, the term "secondary expandability" as used herein means expandability in the reexpansion of the foamed particles, which occurs at the time the foamed particles are filled into a mold and heated with a heating medium, thereby fusion-bonding the foamed particles to one another. The third point of difference is that in the case where the polyolefin resin is used as a base resin, the shrinkage of a molded article obtained after molding under heat becomes more marked than the case where the polystyrene resin is used.
These points of difference make it difficult to integrally mold the foamed particles of the polyolefin resin by filling them into a mold and heating them. Therefore, in order to enhance the secondary expandability of the foamed particles which are starting materials for the production of an expansion-molded article, there has been need for a pretreatment such as a pressurizing treatment of the foamed particle in a pressure tank, and besides it has been necessary to improve a mold and/or devise a heating method in order to improve the fusion bonding property of the foamed particles, or to investigate aging conditions in order to recover the shrinkage of a resulting molded article. With respect to the base resin of the foamed particles, there has also been need for regulation in due consideration of the secondary expandability of the foamed particles, the shrinkage of the molded article, etc. In particular, when it has been necessary to ensure that open voids are provided, and enhance the fusion bonding property of the foamed particles, the above-mentioned points of difference between the foamed particles of the polyolefin resin and the foamed particles of the polystyrene resin have become a serious problem upon molding. Therefore, there has been need to reconsider all aspects including the shape of the foamed particles, heating conditions, secondary expandability, fusion bonding property and the like. Accordingly, it has heretofore been believed that difficulties are encountered on the provision of an expansion-molded article of the polyolefin resin, which is satisfactory in both void content and fusion bonding property among the foamed particles of the polyolefin resin, by a process of filling the foamed particles into a mold to mold them under heat.